Yersinia pestis causes a zoonotic disease, plague, with well established reservoirs on every inhabited continent except Australia. Although humans are an accidental, dead-end host, bubonic and pneumonic plague has caused widespread loss of human life during recurrent pandemics. Nearly 2,000 human plague cases occur yearly and outbreaks in Africa, South America and Southeast Asia are common. In addition, Y. pestis is a category A select agent with potential use for bioterrorism. The ability of pathogens to acquire iron from their hosts is a critical requirement for the development of nearly all infectious disease processes. Y. pestis encodes a large number of proven or putative iron transport systems but the siderophore- dependent yersiniabactin (Ybt) and Yfe systems are the most important for causing bubonic plague. In a mouse model of bubonic plague, the Ybt system is essential in the early phases of disease while the Yfe system plays an important role in the later stages of plague. Mutations in ybt but not yfe affect the pathogenesis of pneumonic plague. Our studies indicate that Yfe and Feo (a ferrous iron transporter) play somewhat redundant roles in iron uptake - mutants lacking both systems show more severe phenotypes for in vitro growth, intracellular growth, and pathogenesis in bubonic and pneumonic plague. All these genes are regulated by Fur and iron availability. In addition, the Ybt system is activated by the AraC-type regulator YbtA and the Ybt siderophore. The ultimate goals of my laboratory are to 1) genetically and biochemically characterize the iron transport systems of Y. pestis; 2) determine the mechanisms regulating expression of these systems; 3) assess the relative importance of these systems in bubonic and pneumonic plague; and 4) explore the potential for new antimicrobial therapies and vaccine components based on these transport systems. In this proposal we will focus on the Ybt system. The specific aims of this proposal are to 1) characterize Ybt regulation, 2) investigate Ybt transport and iron utilization, and 3) examine Ybt roles in bubonic and pneumonic plague pathogenesis.